1. Field of the Invention
The present invention relates to a wireless power transmission system for transferring energy in order to charge a device with electric power or supply or collect electric power to/from a device and also relates to a wireless power transmission unit for use in such a system.
2. Description of the Related Art
Wireless energy transfer technologies have attracted increasing attention these days in order to increase the mobility and water resistance of various electronic devices as much as possible; realize a wireless wall-mounted TV monitor; and charge and supply power to a big moving object such as an electric vehicle.
As a novel non-contact power transmission method that would replace the conventional one that uses electromagnetic induction, a resonant magnetic coupling method has been proposed in United States Patent Application Laid-Open Publication No. 2008/0278264-A1, for example (see FIGS. 6 and 31). According to such a method, resonant mode coupling between resonant antennas is used to realize longer range and higher efficiency power transmission than the conventional electromagnetic induction method. It is thought that particularly if a resonant magnetic field is used, influence on surrounding organisms would be reduced compared to a situation where a resonant electric field is used.
The resonant magnetic coupling method will be described in further detail. As disclosed in Paragraph #0033 of the description of United States Patent Application Laid-Open Publication No. 2008/0278264, the self-resonance phenomenon, which is unique to a loop coil with two open ends, may be used to form a resonator. Alternatively, as disclosed in Paragraph #0038 of that US patent application publication, the resonator may also be formed as a coil loaded with a capacitor. Such a resonant magnetic coupling power transmission system may be regarded as an electromagnetic induction system that does not have a coupling coefficient k of one and yet has its transmission efficiency increased by using a high-Q circuit. Nevertheless, since the coupling coefficient is low (k≠1), high-efficiency energy transfer cannot be realized unless impedance matching is maintained between the power transmitter and the power transmitting resonator, and between the power receiver and the power receiving resonator, which is a restriction that is not imposed on the electromagnetic induction method in which k≈1.
On the other hand, there are many electronic devices or appliances to be driven with a constant voltage, even though that is not the case with some other devices (such as optical devices) that operate in a constant current mode. Likewise, the majority of AC outlets and power supply circuits such as solar cells and fuel cells supply an almost constant voltage. That is why if such a resonant magnetic coupling energy transfer system could be modified so as to cope with a variation in impedance, that system would be applicable to a lot of power sources and a lot of loads. For that purpose, it is preferred that the power transmitting block and power receiving block also have the ability to cope with an impedance variation. To realize that, a wireless section that includes at least a pair of resonators should have variable functions that would allow the system to not only cope with the impedance variation but also maintain the high efficiency transmission ability on a situation basis.
Meanwhile, Japanese Patent Application Laid-Open Publication No. 2007-142418 (see FIG. 3, in particular) discloses an exemplary configuration for a variable inductor.
Furthermore, Japanese Patent Application Laid-Open Publication No. 2001-344574 (see FIG. 3, in particular) discloses a configuration for a variable resonator that achieves a variable resonant frequency characteristic by selecting one of a group of capacitors with mutually different capacitances, which are arranged in parallel with each other, as the one to be connected to an inductor.
However, if the system is tentatively designed so as to cope with those multiple different operating conditions at the same time, it is still difficult to maintain expected high transmission efficiency. Nevertheless, to avoid such a difficulty, the overall length of wiring used should be increased. To overcome such problems with a resonant magnetic coupling wireless power transmission system, a variable resonator, including a variable inductor circuit, is introduced according to the present invention.
It is therefore an object of the present invention to provide a wireless power transmission system that can maintain as high transmission efficiency as possible even under the multiple conditions without unwanted increase of the overall length of wiring.